After further checking, it turns out you cannot currently turn off the autosizing that is sometimes applied by the collision avoidance algorithms. However, there are a couple of workarounds for your situation. I'm not sure which would work best for you, as I have not seen you graph to know the density of points and labels.
Specify a position for the data labels -- if you specify a position for the data labels, then the size will be maintained. However, you might get some collisions, based on the point density.
Use a TEXT plot to do the labeling and use a column to specify the positions -- This will require more work, but it will give you the control to position each label anywhere around the point you want. On the TEXT statement, the POSITION option can take a literal value or a variable containing the valid position values. I would start by giving all points the same position value in the data. Then, adjust the ones that have a collision.
Hope this helps!
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